The present invention relates generally to direct current (dc) motor drives and more particularly to a method of controlling a high performance dc motor operating in an essentially no-load (no torque) mode of operation so as to position the drive to be ready to be able to rapidly respond to demands for high torque without causing undue surges in motor current.
There is, of course, a large variety of dc motor drives many of which use controlled rectifier (e.g., silicon controlled rectifier) bridges to supply electrical current to the motor. When bi-directional motor operation is desired, two such bridges (usually called the forward and reverse bridges) connected in an anti-parallel relationship are often used so that current can be supplied to the motor in either direction in accordance with the direction of motor rotation desired.
An additional common feature in dc motor drives of the type being described is the use of an integrating circuit which receives a command signal representing the desired level of motor operation and a feedback signal representative of the motor terminal voltage. These two signals are combined and the integrated output of the combination serves as the signal for controlling the electrical power furnished to the motor. This integrating assists in stabilizing the system and tends to limit the rate of change of motor current. While the use of the integrating circuit alone does, in the overall sense, lend stability to the system, it also results in a form of instability when the motor is very lightly loaded (i.e., providing little or no torque to the load). The condition which can exist in this light load situation is that the integrating circuit has a tendency to drift and thus provide erroneous command signals to the bridge control circuits. One known way of preventing this drift is to program the firing (rendering conductive) of the bridge rectifiers so that very small amounts of current are furnished to the motor alternately from the forward and reverse bridges. This alternate firing of the forward and reversed bridges adequately stabilizes the integrator and works well for low performance systems. In high performance systems, however, where rapid changes in motor operation are required, problems can develop in this system. This is particularly true in digital type systems where decisions as to which bridge will next be fired must be made in sufficient time to permit the calculation of the appropriate firing angles. When the bridge is in this alternating firing mode, the problem which occurs is that the voltage command signal, which governs the firing circuit, is near a zero torque level until such time as increased torque is required in the motor. If the firing circuit which controls the actual conduction of the bridges has just committed to firing cells in a direction opposite to that which the command signal will be requesting, it will not be able to respond to the actual command voltage until another firing occurs in the committed direction. That is, the bridge representing the committed direction would first fire before the bridge which provides current in the desired direction would be permitted to fire.
During this interim period, the integrating circut earlier described would be continuing its integration process and the relatively long period involved might well be such that the integrated value would be sufficiently large to command an excess current from the firing of the desired bridge. Even if the resulting current were not large enough to cause damage by blowing fuses or destroying bridge thyristors, it might cause a current response which would command a lower current throwing the overall system into an oscillatory condition which may eventually cause a fault due to overcurrent.